An engine may be supplied different types of fuel during different engine operating conditions to enhance engine performance and/or fuel economy. For example, an engine may be supplied gasoline via a first fuel injector and ethanol via a second fuel injector. Gasoline may be the sole fuel supplied to the engine at lower engine loads where the possibility of engine knock may be reduced. As engine load increases, ethanol begins to be supplied to the engine in increasing amounts. Ethanol is also supplied as a greater fraction of fuel provided to the engine so that the possibility of engine knock may be reduced. However, vehicle owners may not be willing to fill a vehicle with two types of fuel to obtain the benefits of operating an engine with two distinct and separate fuels.
One way of supplying two different types of fuel to an engine via refilling a single fuel tank is to separate fuels from a mixture of fuels via a selectively permeable membrane. A fuel mixture comprising two or more fuel types may be exposed to one side of a fuel separating membrane. A pump increases the pressure of the fuel mixture to increase the quantity of higher octane fuel that may be separated or extracted from the fuel mixture. After the fuels are separated the fuels may be stored in separate fuel tanks. However, operating the pump to separate the two fuels increases parasitic losses in the vehicle and the higher and lower octane fuels may recombine via the fuel vapor management system.
Two separated fuels stored in separate tanks may recombine via diurnal heating and cooling of the fuel system. United States of America Patent Publication 2008/000633 describes a way to handle fuel tank vapors from multiple fuel tanks. However, in the system described by publication 2008/000633 fuel vapors of higher octane fuels may condense in fuel tanks holding lower octane fuel. Therefore, additional parasitic energy may be needed to once again separate the higher octane fuel from the lower octane fuel so that engine performance and fuel efficiency may be achieved via the two different fuel types.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for operating an engine, comprising: separating higher octane fuel vapors from a first lower octane fuel; storing the separated fuel vapors in a first fuel vapor storage canister; and limiting the separated fuel vapors from re-entering the first fuel tank holding the first lower octane fuel while not limiting the separated fuel vapors from entering a second fuel tank holding a higher octane fuel.
By separating higher octane fuel from a lower octane fuel mixture and preventing the higher octane fuel from recombining with the lower octane fuel mixture, it may be possible to reduce parasitic losses associated with separating higher octane fuel from a lower octane fuel mixture. Additionally, it may be possible to separate higher octane fuel from a lower octane fuel mixture via diurnal heating without having to recombine the high octane fuel with the lower octane fuel mixture during diurnal cooling so that the higher octane fuel may be separated from the lower octane fuel mixture indefinitely. Consequently, it may be possible to use diurnal heating and cooling to reduce parasitic losses that may accompany separating two types of fuel.
The present description may provide several advantages. For example, the approach may reduce parasitic engine losses that decrease engine fuel economy. Additionally, the approach may provide for more efficient use of fuel vapors. Further still, the approach may be applied to a wide range of fuel system configurations.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.